1. Field of the Invention
This invention relates a collimator lens which is used in an erasable and re-recordable magneto-optical disk system.
2. Description of the Prior Art
The collimator lens of the erasable and re-recordable magneto-optical disk system has a large numerical aperture in order to effectively use divergent light emitted from a light source. The collimator lens is required to provide a short distance from the light source to an outgoing surface of the collimator lens in order to enable a design for making a whole optical system compact.
Japanese Laid Open Publication No. Sho 62-245212 discloses a lens which is designed under the aforementioned object.
The lens mentioned in this publication is a two group three piece construction having a first group which consists of a positive lens and second group which consists of a cemented lens of positive and negative lenses. Furthermore, the first lens group is formed of high refractive index glass material in order to abate the generation of spherical aberration therein.
Though aforementioned prior art collimator lens can be compact in construction, it does not sufficiently correct a chromatic aberration.
The optical system of the erasable and re-recordable magneto-optical disk system includes a light source, a collimator lens for collimating the divergent luminous flux, and an objective lens for converging the light beam onto the recording surface of the optical disk. The objective lens is mounted on an actuator designed such that the objective lens can be finely moved at least in the optical axis direction so that defocusing caused by warping of the disk, etc. can be corrected.
If the collimator lens has a chromatic aberration, when a light source, where the wavelength changes due to change of output, temperature, or the like, is used, even if the aforementioned autofocus mechanism acts normally, the light converging position is not coincident with the recording surface of the disk.
The amount of out of focus P indicated as a following equation; EQU P=(m.sup.2 .multidot..delta.)/2
In this equation, m denotes imaging magnification of an image of the light source which is formed by the collimator lens and the objective lens, and .delta. denotes a chromatic aberration on an axis of the collimator lens.
As mentioned above, when the optical system is used for recording, it is necessary that a numerical aperture NA of the collimator lens be larger in order to utilize energy of a light beam from the light source. Therefore, as the magnification m of the collimator lens grows larger, out of focus is generated by the chromatic aberration.